The present invention relates to jigs and guides for hand-held tools such as saws, routers and the like, and more particularly to jigs for circular cutting.
There are many situations in which it is desired to make a circular cut in a workpiece such as a plywood panel. Such cuts may be made, for example, by marking the panel at a desired radius from a predetermined origin using a compass, and then manually guiding the saw to follow the line. This method, in addition to requiring extra labor, has the obvious disadvantage of inaccuracy and unevenness of the resulting cut due to uncertainties in manually guiding the saw.
In some situations that are encountered in building construction, it is desired to make an arch-shaped cut that is centered precisely at an edge of the panel, such as for making window openings in building walls. In these situations a worker must take extra time to provide a temporary surface extending beyond the edge of the panel and locate the compass point on the temporary surface in line with the edge of the panel. Alternatively, the worker may save time by locating the compass point slightly within the panel, but the accuracy of the resulting cut is further compromised.
U.S. Pat. No. 5,016,354 to Baine discloses a device including an adjustment bar that is rigidly mounted to the base of a saw, another bar that is adjustably clampable to the adjustment bar having an opening for receiving a nail, the nail to be driven into the panel for establishing one end of a cutting radius, the radius being determined by the clamped connection between the bars. While enabling circular cutting of the panel without manual guidance of the saw, the device of Baine is nevertheless subject to certain disadvantages; for example:
1. It is ineffective for arch cuts centered at an edge of the panel in that the nail cannot be driven into the panel precisely at the edge; and
2. It is awkward to use in that a desired cutting radius must be established by trial and error, cutting must be in a single direction (counterclockwise), and the nail is easily lost or damaged.
Thus there is a need for a jig for making arch-shaped cuts that avoids the disadvantages of the prior art.
The present invention meets this need by providing a jig for a hand-held saw having a powered blade, a platform for movably supporting the saw on a face of a workpiece being cut by the blade, and a guide holder for attaching the jig, the jig having edge guide in fixed relation to the platform for contacting an edge of the workpiece, that can easily establish a cutting radius originating at a fixed distance from an edge of the workpiece. In one aspect of the invention, the jig includes a trammel arm having first and second ends, the first end being formed for fixably engaging the guide holder; a clamp assembly fixably locatable on the workpiece and having an edge surface for contacting the edge of the workpiece, the clamp assembly defining a pivot axis extending parallel to the edge surface, the pivot axis being oriented perpendicular to the face when the clamp assembly is fixably located on the workpiece; a pivot member rotatably connected to the clamp assembly on the pivot axis; a fastener for rigidly connecting the pivot member to the trammel arm, the arm being pivotally connected to the clamp assembly on the pivot axis, whereby the saw is guided in an arc concentric with the pivot axis, the pivot axis being at a predetermined distance from the edge of the workpiece.
The pivot axis can be aligned with the edge surface, whereby the predetermined distance is zero when the edge surface is against the edge of the workpiece. Preferably the pivot member is connectable to the trammel arm at a plurality of locations thereon for defining corresponding radii of the arc. Preferably the clamp assembly includes a frame including a planar face member for contacting the face of the workpiece, a web or edge member forming the edge surface and extending in a plane perpendicular to the face member, the pivot member projecting from the face member opposite the web member. The pivot member can have a shank portion and a head portion, the shank portion threadingly engaging the fastener, the trammel arm having a longitudinal slot formed therethrough for receiving the pivot member, whereby the pivot member is clamped at a desired location along the slot.
The head portion of the pivot member can engage the face member, a spacer element contacting the trammel arm on opposite sides of the pivot axis and being clamped against the head portion by the fastener. The slot can have a longitudinal axis that is preferably aligned with the blade when the trammel arm is engaging the guide holder whereby, when the workpiece is being cut, the cutting occurs in line with the longitudinal axis. The trammel arm can have indicia thereon for indicating a radial distance from the blade to the pivot axis. Preferably the first end of the trammel arm is receivable in the holder from opposite sides of the saw for permitting clockwise and counterclockwise cutting, indicia counterparts being on opposite faces of the arm for indicating the radial distance during both the clockwise and the counterclockwise cutting.